DJango query omtimization for foreign key - django

Question

class MainCategory(models.Model):
main_category_id = models.PositiveSmallIntegerField(primary_key=True)
name = models.CharField(max_length=30)
image = models.URLField(null=True)
class Meta:
db_table = 'lookup_main_category'
def __str__(self):
return self.name
class SubCategory(models.Model):
sub_category_id = models.PositiveSmallIntegerField(primary_key=True)
main_category = models.ForeignKey(MainCategory, on_delete=models.CASCADE)
name = models.CharField(max_length=50)
image = models.URLField(null=True)
class Meta:
db_table = 'lookup_sub_category'
def __str__(self):
return self.name
I have the above 2 models in my DJango REST framework project and I need an API output as below. What will be the optimum query without having typical for loop to get distinct main categories and then loop through it in the sub_category table?
[
{
"main_category_id": "10",
"main_category_name": "main_name1",
"image": "http://example/com1",
"sub_categories": [
{
"sub_category_id": "20",
"sub_category_name": "sub_name1",
"image": "http://example/com1"
},
{
"sub_category_id": "21",
"sub_category_name": "sub_name2",
"image": "http://example/com"
}
]
},
{
"main_category_id": "11",
"main_category_name": "main_name2",
"image": "http://example/com2",
"sub_categories": [
{
"sub_category_id": "22",
"sub_category_name": "sub_name2",
"image": "http://example/com2"
}
]
}
]

Answer 1

In this case you would take advantage of prefetch_related, which performs two SQL queries and does an in-memory JOIN.
queryset = MainCategory.objects.prefetch_related('subcategory_set')
Then you iterate over this queryset serializing each object with it's subcategories without hitting the DB.
More info. on official docs here: https://docs.djangoproject.com/en/3.0/ref/models/querysets/#prefetch-related

Answer 2

Finally this is how I managed to get the expected result.
Trick was to use the _set.all()
Official docs are here.
categories_result = []
main_categories = MainCategory.objects.all()
for main_category in main_categories:
main_cat_data = MainCategorySerializer(main_category, many=False)
sub_categories = main_category.subcategory_set.all()
sub_categories_data = SubCategorySerializer(sub_categories, many=True)
result_info = {'main_category': main_cat_data.data, 'sub_categories': sub_categories_data.data}
categories_result.append(result_info)